1. Field of the Invention
The present invention relates to a device for detecting a failure in a piezoelectric apparatus. The device according to the present invention is applicable to a piezoelectric apparatus used as a driver of a wire dot printer in the field of computers.
2. Description of the Related Arts
For driving a piezoelectric element of a piezoelectric apparatus, a voltage pulse produced by the secondary circuit of a transformer is applied to the piezoelectic element. The current of the primary circuit of the transformer is intermittently interrupted in accordance with drive signals to produce the voltage pulse in the secondary circuit of the transformer.
To drive the piezoelectric element through the circuit of the transformer, a primary semiconductor element circuit is connected to the primary winding of the transformer and a secondary semiconductor element circuit is connected to the secondary winding of the transformer. A power source is connected to the primary winding of the transformer.
A drive signal S.sub.1 is applied to the control terminal of a semiconductor element in the primary semiconductor element circuit. An intermittent current flows through the primary winding of the transformer in correspondence with the drive signal S.sub.1, thus causing a voltage pulse to be induced in the secondary winding of the transformer, and the induced voltage pulse is applied to the piezoelectric element through a Zener diode in the secondary semiconductor element circuit to charge the piezoelectric element.
A discharge control signal S.sub.2 generated in synchronization with the drive signal S.sub.1 is supplied to the control terminal of a semiconductor element in the secondary semiconductor element circuit, to cause the energy induced in the secondary winding of the transformer to be released to the ground side to thus discharge the piezoelectric element.
As the result of the above-described charging and discharging, a mechanical distortion or mechanical vibration is produced, and the produced mechanical distortion or mechanical vibration is applied to a load device.
The equivalent circuit of a piezoelectric element is represented by an electrostatic capacitance, and a problem arises in that this electrostatic capacitance, in manufacturing the piezoelectric element, cannot be constantly maintained, and that a failure when open, i.e. interruption of electrical conduction, or a short-circuit is apt to occur, particularly in the case of a small sized piezoelectric element.
In a piezoelectric apparatus with a drive circuit, it is possible for a failure to occur in an open state in the semiconductor element in the secondary semiconductor element circuit, in the piezoelectric element, or the like. When such a failure occurs, the secondary circuit of the transformer of the piezoelectric apparatus opens, and thus the energy stored in the primary circuit of the transformer cannot find a location in the secondary circuit of the transformer and cannot be transmitted, and accordingly, the stored energy is consulted in the semiconductor element in the primary semiconductor element circuit.
Under such circumstances, if the drive circuit of the piezoelectric apparatus is kept in the operating state for a long period when the secondary circuit of the transformer is open, an abnormal heating of the semiconductor element in the primary semiconductor element circuit will occur, which can cause a permanent breakdown of the semiconductor element in the primary semiconductor element circuit.
In a piezoelectric apparatus with a drive circuit, a short circuit in the piezoelectric element can also occur. At this time, the secondary circuit of the transformer is short-circuited directly to the ground only through the internal resistance of the secondary winding of the transformer, other small resistances, and the like.
Under such circumstances, if a driving state continues such that a current exceeding a nominal current flows through the semiconductor element in the secondary semiconductor element circuit, a breakdown of the semiconductor element in the secondary semiconductor element circuit will occur, and an abnormal heating of the primary circuit of the transformer due to the excessive current will also occur.
The delay of the detection of the above-described failure in the piezoelectric apparatus would not only prevent the function of a load device to which the mechanical distortion or mechanical vibration is applied, but would also cause an undesirable influence on the peripheral devices, such as abnormal heating. Therefore, a delay in the detection of a failure is undesirable.